The present invention relates generally to refiners for lignocellulosic material. More particularly, the present invention relates to refiner plate segments for such an apparatus.
In high consistency mechanical pulp refiners, the refining process has a principal goal of separating the lignocellulosic material into individual fibers and giving to these fibers certain mechanical and physical properties which will make them suitable for use in paper, board, building materials, and other products. The wood fibers are worked between two relatively rotating discs on which refiner plates are mounted. The plates usually include a primary refining zone having radial bars and grooves. Due to the large amount of energy transferred to the fiber in the primary refining process, a portion of the moisture content of the feed material is vaporized into steam. This steam separates into a xe2x80x9cforward flowingxe2x80x9d proportion, which will flow out with the refined fiber, and a xe2x80x9cback flowingxe2x80x9d proportion, which will flow back towards the refiner inlet.
The feed material is generally wood chips, wood particles, or wood debris from various sources. Generally, some degree of thermal softening of the wood fibers in the feed material is deemed necessary to allow the fibers to be in optimal condition for the primary refining operation. The back-flowing steam from the primary refining zone is generally the principal source of heat for the thermal softening of the feed material. It is therefore necessary to control the back flowing steam to ensure that a sufficient supply of steam is available to condition the feed material while preventing the back flowing steam from interfering with the stability of the feed.
Most conventional refiner plates fail to properly break down the feed material prior to the primary refining action. Generally, the thermal softening of the fibers in the feed material in these refiner plates is not consistent due to the non-uniform and relatively large size of the feed material particles and the limited period of time for conditioning. Those refiner plates which break down the feed material properly suffer from a lack of control of the feeding intensity, lack of proper feed distribution and/or an increased negative interaction between back-flowing steam and feed.
Briefly stated, the invention in a preferred form is a refiner plate segment for a refiner having opposed first and second refiner plates. Each refiner plate segment has radially inner and outer ends, multiple radially disposed bars, and grooves alternating with the bars. Each of the grooves defines a base having a radial length. The base of at least one of the grooves has a variable base profile along the radial length of the base such that in at least one radial position between the inner and outer ends of the refiner plate segment, the base profile of the refiner plate segments of the first refiner plate has a low point and the base profile of the refiner plate segments of the second refiner plate has an oppositely disposed high point. The high point in the base profile forces lignocellulosic material carried on the second refiner plate onto the first refiner plate.
For refiners having counter-rotating refiner plates, a single transfer of the material from one of the rotating refiner plates to the counter-rotating refiner plate, as described above, may suffice to provide all of the benefits of the subject invention. However, for xe2x80x9csingle diskxe2x80x9d refiners (where there is only one rotating disk) transfer of the material back to the rotating disk is required to prevent stalling of the material on the stator disk. The refiner plate segments for such refiners therefore have three radially separated positions for transferring material between the stator and rotor plates. At a first radial position between the inner and outer ends, the base profile of the refiner plate segments of the first refiner plate has a low point and the base profile of the refiner plate segments of the second refiner plate has an oppositely disposed high point. At a second radial position between the outer ends and the first radial position, the base profile of the refiner plate segments of the first refiner plate has a high point and the base profile of the refiner plate segments of the second refiner plate has an oppositely disposed low point. At a third radial position between the outer ends and the second radial position, the base profile of the refiner plate segments of the first refiner plate has a low point and the base profile of the refiner plate segments of the second plate refiner has an oppositely disposed high point. Material is transferred from the second (stator) plate to first (rotor) plate at the first position, from the first plate to the second plate at the second position, and from the second plate to the first plate at the third position.
Using the base profile to control the interaction between the feed material and the stator and rotor plates allows the design of the bars to be customized to further improve the performance of the refiner plates. For example, the height of the bar of the refiner plate segments of the rotor plate may be made greater than the height of the bars of the refiner plate segments of the stator plate to increase the feeding effect of the rotor plate. The feeding effect of the rotor plate may also be increased by making the length of the bars of the refiner plate segments of the rotor plate may be made greater than the length of the bars of the refiner plate segments of the stator plate. The draft angle of the bars of the refiner plate segments of the stator plate may be made greater than the draft angle of the bars of the refiner plate segments of the rotor plate to increase the tangential frictional effect of the rotor bars.
It is an object of the invention to provide a refiner plate which partially separates and reduces the size of the feed material to a primary refiner plate.
It is also an object of the invention to provide a refiner plate which optimizes the feeding characteristics of the feed material to a primary refiner plate.
It is further an object of the invention to provide a refiner plate which evenly distributes feed material around a primary refiner plate.
It is still further an object of the invention to provide a refiner plate which promotes backflow of a quantity of steam appropriate for conditioning the feed material while minimizing the interaction between the steam and the feed material.
Other objects and advantages of the invention will become apparent from the drawings and specification.